Field of the Invention
The invention relates to a method for operating a fuel cell stack for a fuel cell system, in which by reversing the flow direction of a coolant during a cooling operation, the coolant in the fuel cell stack is initially conveyed in a first direction and is subsequently conveyed in a second direction which is at least substantially opposite to the first direction. The invention further relates to a fuel cell system.
Description of the Related Art
When a fuel cell stack of a fuel cell system is put in operation at cold ambient temperatures, i.e., temperatures less than 0° C., heat generation typically occurs at isolated locations in the fuel cell stack, as the result of which the reaction of hydrogen with oxygen is initiated particularly quickly at these locations. With increasing heating of the location, increasingly more electrical energy can be delivered from this area of the fuel cell, and the location heats up further. Since the efficiency of the fuel cell rises with increasing temperature, even more electrical energy can be generated, and the location heats up even further.
These types of locally delimited locations, having a higher temperature than their surroundings in the fuel cell stack, are also referred to as “hot spots.” Since excessive heating of the fuel cell stack in the area of the hot spots may dry out or damage the membrane of the fuel cell, even for a cold start or a freezing cold start of the fuel cell stack a coolant flow is set which dissipates the heat from the hot spots. Thus, even in a cold start or freezing cold start of the fuel cell stack, a cooling operation takes place which is used for dissipating the heat generated by the hot spots.
DE 10 2007 034 300 A1 describes a reversal of the flow direction of the coolant in the fuel cell stack during start-up of the fuel cell stack. A coolant pump which is able to switch over from a first conveying direction to a second conveying direction opposite to the first direction is situated in a coolant circuit which encompasses the fuel cell stack. For reversing the flow direction, a comparatively short cycle time of six seconds, for example, is selected in order to keep temperature differences within the fuel cell stack as low as possible. An even shorter cycle time may be set in order to further decrease the temperature range that is present in the fuel cell stack.
The fact that problems in the cooling operation may result despite the reversal of the flow direction of the coolant is regarded as disadvantageous.
The object of the present invention, therefore, is to provide a method of the type stated at the outset as well as a fuel cell system by means of which an improvement in the starting behavior of the fuel cell stack may be achieved.